Each location has to pick one of the Z decay exercises:
DELPHI ( Hands-on-CERN   or   Keyhole ) or
OPAL (Identifying particles).

It would be very desirable, if we had each day
both, DELPHI and OPAL events covered:
=> please choose according to the available translations


INTRODUCTION:
=============
In the 1/2h introduction to the exercise (which can be done e.g. by browsing together through the first pages of the exercise package using a beamer) the scientist should address all important issues, but can for sure not cover all eventualities (e.g. muons appearing in jets, converted photons from pi0 on tau decays, etc.). The students
should encounter problems themselves. Especially the "tau-tau" class should not just used as "all what doesnt fit elsewhere", but the students should be convinced about their choice. 

The presence of graduate student tutors for helping the high school
students with their PC exercises is extremely important. For about each
10 students (5 groups) one graduate student as tutor is needed.

CONDUCTING EXERCISES:
=====================

Each exercise package contains 1000 events which should be 
distributed amongst the students, refer to the online file packages:
For 10 groups a 2 students: each group analyses 100 events
(see Zdecay_xxxx100.xls, xxx=DELPHI/OPAL local combination files)
For 20 groups a 2 students: each group analyses 50 events
(see Zdecay_xxxx50.xls, xxx=DELPHI/OPAL local combination files)
any other distribution is also possible, however, data sizes smaller than 50 events/ group are not recommended (too few leptons, statistical error will dominate).

If identical data samples are given to different groups, their results should be averaged, before entering the local EXCEL table.

NB: the EXCEL file does *not* require that the groups work through all of their events. Often some careful groups are much more slower than the others. Then either the faster groups have to get another voluntary exercise or the slower groups just give e.g. the result
for 70 scanned events instead of 100.  

STRATEGY:
=========
Identifying Z0 decays can be made easier by using conservation laws (most evidently charge and energy!). 
Try to orient your reconstruction strategy on the examples given: 

are leptons contained in the event (if yes, which? - how many?)
are jets contained in the event  (if yes, which? - how many?)

Prevent the students to put all difficult decays into the tau-tau class. make yourself familiar with rules of thunmb for difficult decays, e.g. if a jet contains roughly 3 tracks it is mostly a tau jet

IMPORTANT:
==========
Safe your local .xls data file and transfer the data to the moderator before the video conference via koala!!


DETAILS:
========

Please note that OPAL data have a cos theta cut in order
to suppress Bhabha e+e- scattering w.r.t. Z0 decays to e+e-. An extra correction factor of 1.6 is applied in Zdecay_OPAL.xls to account for this cut. No such correction has to be applied to DELPHI data, which cover a smaller angular range (Zdecay_DELPHI.xls)
and are thus less exposed to Bhabha background.

The decay probabilities ("branching ratios") are calculated in the excel files w.r.t the *visible* decays. They are not corrected for the 20% invisible Neutrino decays of the Z. For the discussion,
(see text file on discussion) the neutrino decays are irrelevant.

The theoretical results have been taken from R_e, R_mu and R_tau in table 2.13 in physics reports 427, (2006) 257-454. (R =N(ll)/N(qq) after all radiative corrections, no correction for phase space effects of the tau mass => R_tau small)

GOALS:
======